1. Field of the Invention
The present invention relates to an R-T-B based sintered magnet for use to make motors for cars and a method of producing such a magnet.
2. Description of the Related Art
An R-T-B based rare-earth sintered magnet, including an Nd2Fe14B type compound phase as a main phase, is known as a permanent magnet with the highest performance, and has been used in various types of motors such as a voice coil motor (VCM) for a hard disk drive and a motor for a hybrid car and in numerous types of consumer electronic appliances. When used in motors and various other devices, the R-T-B based rare-earth sintered magnet should exhibit thermal resistance and coercivity that are high enough to withstand an operating environment at an elevated temperature.
To increase the coercivity of an R-T-B based rare-earth sintered magnet, an alloy, obtained by mixing together not only a light rare-earth element RL but also a predetermined amount of heavy rare-earth element RH as rare-earth elements R in the material and then melting the mixture, has been used. According to this method, the light rare-earth element RL, which is included as a rare-earth element R in an R2Fe14B main phase, is replaced with the heavy rare-earth element RH, and therefore, the magnetocrystalline anisotropy (which is a decisive quality parameter that determines the coercivity) of the R2Fe14B phase improves.
However, although the magnetic moment of the light rare-earth element RL in the R2Fe14B phase has the same direction as that of Fe, the magnetic moments of the heavy rare-earth element RH and Fe have mutually opposite directions. That is why the remanence Br would decrease in proportion to the percentage of the light rare-earth element RL replaced with the heavy rare-earth element RH.
A magnet for use in motors, for example, should have not only high remanence BR at least in its portion to be used for a driving section but also high coercivity at least in its portion to be exposed to intense heat or a great demagnetizing field.
For that purpose, according to a conventional technique, a magnet with high remanence Br and a magnet with high coercivity HcJ are bonded and combined together with an adhesive, and a combined magnet thus obtained is used in motors and various other machines. If such a combined magnet needs to be made, however, it takes extra time to complete that bonding process, thus causing a decrease in productivity. What is worse, if a lot of adhesive must be used to bond the two different magnets together, a magnetically discontinuous layer would be formed by the adhesive.
Meanwhile, methods for forming such a combined magnet without using any adhesive have also been proposed in Japanese Patent Application Laid-Open Publication No. 57-148566 and Japanese Utility Model Application Laid-Open Publication No. 59-117281. Specifically, Japanese Patent Application Laid-Open Publication No. 57-148566 discloses a field composite permanent magnet produced by compacting together one material with higher remanence than others and the other material with higher coercivity than others and then sintering the compact.
On the other hand, Japanese Utility Model Application Laid-Open Publication No. 59-117281 discloses field permanent magnets with an arc cross section that together form a permanent magnet for a DC machine. Specifically, in each of those field permanent magnets, only a portion near the surface of its inner arc and around the edge of its inner end surface on the demagnetizing side is designed to be a permanent magnet with higher coercivity than the body permanent magnet.
However, the techniques disclosed in both of these documents are supposed to be used to make ferrite magnets, and will not meet the demands for reducing the size of motors or improving the performance thereof. On top of that, as materials with mutually different compositions are combined together through sintering, such a combined magnet is likely to be deformed during the sintering process. And the higher the temperature at which such a magnet is used, the more easily the magnet will crack from their junction due to a difference in sintering shrinkage rate between those materials.
To make magnets for EPS and HEV motors, for which there should be growing demands from the markets in the near future, R-T-B based sintered magnets with essentially good magnetic properties need to be used effectively. And a lot of people are waiting for development of a technology for making an R-T-B based sintered magnet including both a region with high remanence Br and a region with high coercivity HcJ.